The importance of local neuronal circuits (LNCs) in the central nervous system for information processing has been emphasized repeatedly. Dendrodendritic inhibition is considered a key mechanism in the operation of LNCs and yet, to date, our understanding of this mechanism is limited almost entirely to data based on extracellular field potential recording. These data do not offer the high resolution necessary for a full understanding of this intricate form of cellular interaction. We have developed a turtle in vitro olfactory bulb preparation with which we have already provided the first intracellular demonstration of dendrodendritic inhibition, and have begun to elucidate details of this mechanism. We propose to continue by studying the main cell types which make up the reciprocal dendrodendritic synapse: the mitral and granule cells. We will: 1) analyze the complex orthodromic mitral cell IPSP to determine if a specific origin, and neurotransmitter, can be assigned each component, 2) determine the ways in which reciprocal interactions are modulated by centrifugal afferents and their neurotransmitters, 3) investigate the novel finding that mitral cell somadendritic regions act like presynaptic membranes by comparing their calcium spiking properties with those of other synaptic terminals, 4) determine the electroresponsiveness of granule cell membranes to discover the extent over which granule cell influence can spread and 5) label granule cells with an intracellular dye to verify that the physiological properties are indeed, those of granule cells. These experiments will provide answers to questions concerning the composition, range, mode of action and modifiability of olfactory bulb dendrodendritic interactions. They will not only provide deeper insight into the specific functioning of an important mechanism of local circuit interactions but will have broader implications for understanding the roles these circuits play in information processing in general.